1. Field of the Invention
The present invention relates to a technology for a magnetic detection coil and an apparatus for magnetic field measurement.
2. Description of Related Art
An apparatus for biomagnetic field measurement, which is used for magnetocardiography and magnetoencephalography, has so far employed a technique that uses a magnetic detection coil made of a superconductor wire to detect magnetic signals generated by a living organism and to transmit them to a superconducting quantum interference device (hereinafter referred to as SQUID). A SQUID has a superconducting ring with Josephson junctions, in which a voltage across the Josephson junction cyclically varies with a period of Φ0=h/2e (Wb) according to magnetic flux penetrating the SQUID.
It has been a technique typically adopted in magnetocardiography and magnetoencephalography to use a magnetic detection coil made of a superconductor wire to detect magnetic signals generated by a measurement object as magnetic flux and transmit the detected magnetic flux to a SQUID. The magnetic detection coil functions to eliminate noise caused by environmental magnetic fields so as to increase the Signal-to-Noise (S/N) ratio.
FIG. 9 is a schematic diagram illustrating a composition of a Flux Locked Loop (FLL) circuit in a typical apparatus for magnetic field measurement.
In the FLL circuit 1300, a current produced by magnetic flux penetrating a magnetic detection coil 1301 flows through both a magnetic detection coil 1301 and an input coil 1302. Accordingly, the input coil 1302 generates magnetic flux, which is then transmitted to a SQUID 1303. The SQUID 1303 has a superconducting ring with Josephson junctions. A bias current source 1305 supplies a bias current to the SQUID 1303. A voltage across the SQUID 1303 cyclically varies with a period of Φ0=h/2e (Wb), according to magnetic flux penetrating the SQUID 1303. The FLL circuit 1300 has a feedback circuit as well, which is constituted by a pre-amplifier 1306, an integrator 1307, a feedback resistor 1308 and a feedback coil 1304. This feedback circuit follows the SQUID 1303 in the FLL circuit 1300. The feedback coil 1304 feeds back magnetic flux so as to negate any change in magnetic flux penetrating the SQUID 1303.
It is possible to obtain the current value for the current flowing through the feedback coil 1304 by measuring a potential difference across the feedback resistor 1308. The magnetic flux penetrating the SQUID 1303 can be calculated based on this obtained current value.
A circuit which has the composition described above is called an FLL circuit. The FLL circuit 1300 can provide an output voltage proportional to a magnetic field detected with the magnetic detection coil 1301.
The following description is given of a typical magnetic detection coil used for an apparatus for biomagnetic field measurement, while referring to FIG. 10.
FIG. 10 illustrates schematic diagrams for typical magnetic detection coils used for an apparatus for biomagnetic field measurement. FIG. 10A shows a zeroth-order differential coil (magnetometer). FIG. 10B shows a first-order differential coil, FIG. 10C shows a second-order differential coil. FIG. 10D shows a zeroth-order differential coil formed on a thin film substrate. FIG. 10E shows a first-order differential coil formed on a thin film substrate.
As shown in these drawings, a magnetic detection coil typically employs an arrangement where a superconductor wire is wound around a cylindrical bobbin or another arrangement where thin films are formed on a substrate.
As shown in FIG. 10A, a zeroth-order differential magnetic detection coil 181 has a coil 181a which is made of a superconductor wire wound one turn around a bobbin 1811. Magnetic flux ΦM detected by the zeroth-order differential magnetic detection coil 181 is represented by the following equation (1):ΦM=Φ181a  (1)where Φ181a represents magnetic flux penetrating the coil 181a. 
Because the magnetic flux ΦM is equal to the magnetic flux Φ181a which penetrates the coil 181a, the zeroth-order differential magnetic detection coil 181 functions to yield magnetic signals greater than any of the first-order and the second-order differential magnetic detection coils to be described later. However, the zeroth-order differential magnetic detection coil does not have a function of decreasing an effect of the environmental magnetic field at all. Therefore the zeroth-order differential magnetic detection coil 181 outputs a signal which is directly affected by a noise caused by the environmental magnetic field. Accordingly, the zeroth-order differential magnetic detection coil 181 is usually used in a magnetically shielded room.
In this specification, a distance between centers of coils is referred to as “center-to-center distance”.
Given the magnetic flux ΦM shown in FIG. 10A provides a positive magnetic signal, the zeroth-order differential magnetic detection coil 181 generates a current flowing in the direction indicated by an arrow along the coil as shown in the same drawing. Hereinafter, a magnetic signal corresponds to magnetic flux pointing upward, as shown in FIG. 10A, is defined as a positive magnetic signal. The direction, in which the current flows through a magnetic detection coil for a positive magnetic signal to be detected, is represented by an arrow.
As shown in FIG. 10B, a first-order differential magnetic detection coil 182 comprises coils 182a and 182b. The coil 182a is made of a superconductor wire wound one turn around a bobbin 1821 in the “first winding direction.” The coil 182b also is made of a superconductor wire wound one turn around the bobbin 1821 in the “second winding direction” which is defined as an opposite winding direction to the “first winding direction”, and disposed a predetermined distance “vertically” (as described later) apart from the coil 182a. With the composition described above, magnetic flux ΦG1 detected by the first-order differential magnetic detection coil 182 is represented by the following equation (2):ΦG1=Φ182a−Φ182b  (2)where Φ182a gives magnetic flux which penetrates the coil 182a, and Φ182b gives magnetic flux which penetrates the coil 182b. 
The reason why magnetic flux Φ182b is subtracted from Φ182a is that the coil 182b is wound in the opposite winding direction.
Taking a difference in magnetic field as described above with the equation (2) is referred to as “differentiating” in this specification. For example, “taking a difference once” is referred to as “first-order differentiating”, and “taking a difference twice” is referred to as “second-order differentiating”.
The coil 182a is positioned in the vicinity of a test object in order to detect a magnetic field, while the coil 182b is positioned relatively far away from the test object. As a result, the effect of the environmental magnetic field which is uniformly distributed in the space is negated, and the magnetic flux only attributed to the test object can be detected.
It should also be noted that a “vertical” direction is referred to as a direction perpendicular to the loop plane and that a “horizontal” direction is referred to as a direction in parallel with the loop plane. Accordingly, it may be possible to have the vertical direction coincide with the direction of magnetic field measurement.
As shown in FIG. 10C, a second-order differential magnetic detection coil 183 comprises coils 183a, 183b and 183c. The coil 183a is made of a superconductor wire wound one turn around a bobbin 1831 in the first winding direction. The coil 183b is made of a superconductor wire wound two turns around the bobbin 1831 in the second winding direction, which is opposite to the first winding direction, and disposed a predetermined distance vertically apart from the coil 183a. The coil 183c is made of a superconductor wire wound one turn around the bobbin 1831 in the first winding direction, and disposed the predetermined distance vertically apart from coil 183b. Magnetic flux ΦG2 detected by the second-order differential magnetic detection coil 183 is represented by the following equation (3):ΦG2=Φ183a−2Φ183b+Φ183c  (3)where Φ183a gives magnetic flux which penetrates the coil 183a, Φ183b gives magnetic flux which penetrates the coil 183b, and Φ183c gives magnetic flux which penetrates the coil 183c. 
As described above, by taking two steps to differentiate magnetic flux with respect to the vertical direction, the second-order differential magnetic detection coil 183 is capable of reducing an effect of the environmental magnetic field distributed with a first-order gradient as well as an effect of the environmental magnetic field distributed uniformly in the space. As a result, the second-order differential magnetic detection coil 183 is capable of reducing the effect of environmental magnetic field more efficiently than the first-order differential magnetic detection coil 182, which is only capable of reducing the effect of environmental magnetic field distributed uniformly in the space. However, if magnetic signals are included in magnetic flux Φ183b penetrating the coil 183b and magnetic flux Φ183c penetrating the coil 183c, the second-order differential magnetic detection coil 183 results in a lowered magnetic signal being detected.
There is a technique for forming a magnetic detection coil by using superconductor thin films, instead of a superconductor wire. FIG. 10D illustrates a zeroth-order differential magnetic detection coil 184 constituted by superconductor thin films formed on a substrate 1841. Magnetic flux Φ184a detected by a coil 184a is transmitted to a SQUID 1842 formed on the same substrate 1841. Similarly, FIG. 10E illustrates a first-order differential magnetic detection coil 185 constituted by coils 185a and 185b formed on a substrate 1851, which have winding directions opposite to each other. A magnetic flux difference Φ185a−Φ185b between magnetic flux Φ185a detected by the coil 185a and magnetic flux Φ185b detected by the coil 185b is transmitted to a SQUID 1852 formed on the same substrate 1851. An advantage of using superconducting thin films is that it is possible to fabricate a magnetic detection coil having an accurate magnetic detection coil area as specified.
Based on characteristic features of differential coils described above, several types of arrangements for magnetic detection coils have so far been proposed. As an example of such arrangements, a technique has been proposed as stated in Japanese laid-open patent application 09-084777; wherein plural types of magnetic detection coils having different differential orders were placed at the same measurement point so as to calculate and estimate a magnetic field source and/or a magnetic field source distribution within a living organism.
FIG. 11 gives a perspective view of a magnetic detection coil as specified by Japanese laid-open patent application 2007-108083.
It has proposed a magnetic detection coil, e.g. a coil 200 as shown in FIG. 11, which differentiates gradients in the magnetic field in two different directions. The magnetic detection coil 200 in FIG. 11 comprises second-order differential coils 201a and 201b given by FIG. 10C which are arranged so as to take the difference along the x-axis direction.
However, since the conventional differential coil having a structure which makes it possible to detect magnetic field differentiated in a certain direction, as shown in FIG. 10, said arrangement has a problem if the effects of environmental magnetic fields are strong; for example, in the case of an environment without a magnetic shield, it is not possible to adequately reduce the effects of environmental magnetic fields. One possible technique for decreasing said effects is to increase the order of a differential magnetic detection coil. Although the effects of environmental magnetic fields can be decreased with the technique, magnetic signals to be detected may also be decreased.
As for a technique utilizing a magnetic detection coil formed on superconducting thin films, it has a problem of having a difficulty in fabricating a magnetic detection coil with a three-dimensional structure due to characteristic of the thin films.
It should be noted that there is a trade-off a trade-off between the differential order of the magnetic coils and the signal level from the magnetic coils. If the differential order of the magnetic coils is increased, the signal level from the magnetic coils lowers although the effect of the environmental magnetic field is decreased. Biomagnetic field measurements have so far been carried out usually with first-order differential magnetic detection coils, or second-order differential magnetic detection coils in a magnetically shielded room, depending on the magnitude of an environmental magnetic field.
In addition, since a type of a magnetic detection coil having a three-dimensional structure, as shown in FIGS. 10A to 10C, and a type of a magnetic detection coil formed on superconducting thin films, as shown in FIGS. 10D and 10E, are different from each other in terms of their usages, structures and methods for manufacturing, it appears to be unproductive to get an idea of combining these two types of coils.
According to Japanese laid-open patent application 2007-108083, it has become apparent that signal intensity can decrease according to the direction of a current as the signal source, though the stated technique can more efficiently decrease the effect of the environmental magnetic field than the technique described in Japanese laid-open patent application 09-084777. Namely, the following description continues while referring to FIG. 11; a magnetic detection coil according to Japanese laid-open patent application 2007-108083 gives good detection sensitivity when a current flows in parallel to the x-axis. However, the coil can significantly decrease detection sensitivity when a current flows in parallel to the y-axis, i.e. in the direction perpendicular to the line connecting the respective centers of second-order differential coils which form the magnetic detection coil 200. In addition, this type of a magnetic detection coil cannot sufficiently reduce the effects of environmental magnetic fields if the effects are large.
In short, conventional differential coils have a problem of inability to provide sufficient reduction of environmental magnetic fields if the effects of environmental magnetic fields are large; they also have a problem of causing a decrease in signal intensity while lowering the effects of environmental magnetic fields, depending on the direction of a current used as the signal source.